Previous theories have held that the planet's mysterious northern lowlands, pictured above in blue, once hosted oceans, but the latest climate models reveal that Mars was likely too cold and dry to support such a large body of water.

We've visited the surface of Mars no fewer than half a dozen times since the 1970s, and on every occasion we've encountered a frigid, arid climate. But numerous observations have led scientists to the conclusion that conditions were not always so inhospitable to moisture. Intricate, winding valleys, reminiscent of dried-up river beds, have been found branched across the planet's surface; and clays and mineral deposits — traces many astrogeologists claim could only have formed through hundreds of years of exposure to water — have provided what researchers recently called "bullet proof evidence" of Mars' warm, watery past.

Findings like these have made the idea of an ancient Martian ocean seem more and more plausible in recent years. But planetary scientist Jim Head and others have begun piecing together evidence that suggests the Red Planet has always been frigid and desiccated (at least on the surface); any water that ran on the face of Mars, they argue, would have done so only briefly. Nature's Eric Hand explains:

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Head and his colleagues have challenged the feasibility of a Martian ocean with several related lines of evidence. The first, explains Nature's Eric Hand, "comes from models of the ancient Martian climate that fail to predict temperatures high enough for rain, or for liquid water to persist on the surface at all." He continues:

The young Sun was fainter than it is today, and even if the young Mars had a thicker atmosphere, its greenhouse effect would probably not have warmed the planet above freezing, says François Forget at the University of Paris. He has submitted to the journal Icarus what he says is the most sophisticated three-dimensional climate model for Mars so far. It predicts that any water on Mars would have been bound as ice at higher elevations.

Any valley networks seen on the Red Planet's surface, hypothesize Head and Forget, might have been formed during brief periods of volcanic activity or major impact events, either of which could have warmed the Martian atmosphere long enough to unleash high-altitude ice-stores in the form of liquid water. Bolstering this hypothesis is the observation that many of these valley networks appear to have formed hundreds of millions of years apart.

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What's more, Hand says that even clay minerals may not support a Mars ocean hypothesis:

A team using a spectrometer on the Mars Reconnaissance Orbiter found that roughly 80% of the clays occur together with other minerals that form at relatively high temperatures. This suggests that the clays formed not in cool surface water but underground, in water warmed by leftover heat from Mars's formation, says Bethany Ehlmann, a planetary scientist at the California Institute of Technology in Pasadena, who led the study.

Next month, planetary scientists will convene at a conference, organized by the Lunar and Planetary Institute's Stephen Clifford, to discuss the early climate of Mars. A month after that, NASA's Curiosity rover is slated to land on the surface of the Red Planet, where (if all goes well) it will perform some of the most incisive analytical experiments ever conducted on another cosmic body. Mars' history may be full of secrets, but we're getting closer to unveiling those secrets every day.